It has already been proposed to apply particulate material capable of receiving an electrical charge, especially particulate solids and materials formed by chopping or cutting fibers, filaments or strands and even flocks, to adhesive-coated surfaces so that the applied material appears in relief and in accordance with a particular pattern upon the substrate.
In conventional systems of this type, the substrate is coated in accordance with the desired pattern with a layer of adhesive and the coated surface is disposed beneath a distributor for the electrifiable material. The latter can comprise a vibratory sieve which meters the electrifiable material onto the adhesive-coated surface and which can have a grill at its lower or discharge side which is connected to a terminal of a high voltage electrostatic field generator.
Because of the electrostatic field which is created, the material is generally drawn or directed perpendicular to the grill and to the receiving surface onto the adhesive coating and is implanted or bonded to the latter.
In practice, such sieves may be provided with partitions above the grill to define compartments for electrifiable materials of different colors whereby corresponding color patterns are generated on the adhesive layer in a single operation.
In all of these prior art systems it is necessary to remove excess electrifiable material from the substrate in regions in which this material has not been implanted in the adhesive zones. In practice the cleaning or particulate removal operation, because of the large quantities of material which must be removed is effected by mechanical means, generally a combination of aspiration (suction) and beating or agitation of the substrate.
While one can utilize, in addition or alternatively, an electrostatic system for picking up the excess by electrostatically charged surfaces, generally the electrostatic techniques are less effective. Hence even when electrostatic means are employed it is frequently necessary to complete the cleaning by aspiration and agitation.
In order to avoid damage to the applied pattern, it is customary to carry out the cleaning operations described above only after the implanted particles have been firmly bonded by adhesive to the substrate.
Thus a standard coating method comprises electrostatically applying the particles (i.e. the flock, cut fiber or the like) to a support previously coated with the adhesive, fixing the pattern particles in place by the adhesive to the support or substrate generally by a thermal treatment and agitating the support while drawing off excess particles by aspiration with or without electrostatic cleaning.
Such techniques have numerous disadvantages and primary among them is the dispersion of the particles which occurs upon deposition or removal. In general the dispersion is into the air around the coating and removal zones and the particles may contaminate the work space posing an environmental hazard to operating personnel, or contaminating other parts of the machinery or equipment used.
In the electrostatic application of the particles, the latter are distributed by means of a rectangular grill having an electrostatic charge which is imparted to the particles. When the particles reach or approach the adhesive layer or the substrate, they tend to reverse in polarity and are attracted back to the grill. As a result while there is a net movement of particles from the grill to the substrate, there is significant back and forth movement of particles in the space between the grill and the substrate.
At the center of the grill, where the lines of electrostatic force are substantially vertical and perpendicular to the parallel planes of the grill and the substrate, substantially all of the movement is in the vertical direction along the lines of force.
However, along the periphery of the grill the lines of the electromagnetic force bend outwardly as a result of the magnetic effect of large metal masses surrounding the space between the grill and the substrate and substantially invariable present in a normal workplace. Such neighboring equipment may include silk screen (screen printing or serigraphic equipment) or printing machinery for applying the adhesive or performing other decorative operations.
Because of the outward divergence of the magnetic lines of force, an outward dispersion of the particles can occur to contaminate the associated equipment.
Thus earlier arrangements for applying flock and other particle patterns to a substrate could not effectively be performed in the region of such other equipment without contaminating the latter. It thus was not possible in a single installation to provide an electrifiable material to a support which was imprinted with a color scheme, pattern or even legible matter. In practice, therefore, the adhesive was applied to the support, coated with the particulate material to form the relief pattern and only thereafter, following cleaning, was the support delivered to a printing machine.